The warble is a hairy fly that becomes parasitic on cattle, deer and horses. It is scientifically known as the genus Hypoderma. Hypoderma lineatum is the term for horse warbles, and Hypoderma bovis is the term for cattle warbles. The warble is also known as a “heel-fly,” “bomb fly” and “gad fly” in common language. The larvae are often referred to as “cattle grubs” or “wolves.”

Typically the warble lays its eggs on the cattle’s leg hairs. One fly may lay up to eight hundred eggs on one cow. When the eggs hatch, the larvae crawl down the hair, and onto the skin, penetrating it to get onto the subcutaneous level — the level just below the skin. This is incredibly irritating to the animal.

After months of traveling through the cattle’s body, and growing larger, the larvae typically settle on the cattle’s back, near the spine, and swell. The swelling results in a pimpled cyst under the cattle’s skin. There, the larvae molt for thirty days. Then, a mature larva squirms out of the cyst and falls to the ground to endure a 35 to 60 day cocoon-like pupation stage. After this stage, the adult warble will emerge to begin the cycle again.

Though the warbles do not cause direct harm to the animal, they can pose many harmful effects. Since the fly is so persistent, and the hatched larvae cause so much discomfort to the animal, the animal may not feed properly due to the constant strain. This can result in the animal may lose significant weight, resulting in decreased milk production. Value of the carcass hide can also depreciate when riddled with holes from larvae emerging from cysts.

While warble refers to this type of pesky fly, it also refers to a musical trill or a delightful succession of low notes. The musical definition is derived from the 14th century Old High German word for whirlwind. In this use, the word can also be used as a verb, as in, the act of warbling. Warbling is to sing with many twists and turns; that is, with a variety of pitches that change quickly.

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